The term “photovoltaic” comes from the Greek phos meaning “light”, and “voltaic”, from the name of the Italian physicist Volta, after whom the unit of electro-motive force—the volt—is named. Photovoltaics are, therefore, related to the practical application of photovoltaic cells to produce electricity from light, even though the term is often used specifically to refer to the generation of electricity from sunlight.
Harvesting solar energy using the photovoltaic effect requires active semiconducting materials to convert light into electricity. Currently, solar cells based on silicon are the dominant technology due to their high conversion efficiency. Mono-layered inorganic solar cells are composed of an inorganic semiconductor sandwiched between two metallic electrodes with different electrochemical potentials (Fermi levels), where one of electrodes is semi-transparent or grid-like. They generate voltage when light penetrates into the semiconductor. A photon breaks a covalent bond between silicon atoms and ‘kicks’ one electron out. The atom that loses the electron gets a positive charge and becomes a hole. Further electrons are transported to the anode and holes to the cathode driven by a difference in electrochemical potentials of the electrodes. Mono-layered devices usually possess low power conversion efficiency.
Bi-layered devices are composed of two types of semiconductor: p-type and n-type. In this case, the charge separation occurs near the border between p-type and n-type semiconductors and it is much more efficient, because the charges are ‘captured’ and held strong by ‘host’ semiconductors. Triple-layered and multi-layered devices are also known.
Inorganic solar cells are chemically and thermally stable devices, and power conversion efficiencies of modern inorganic solar cells reach the 30% barrier, and therefore they are relatively efficient. This is a major advantage of inorganic solar cells over organic devices, which have less than a third of the efficiency of silicon-based cells. However, inorganic solar cells also possess disadvantages. Their production is costly and energy consuming because it requires a thorough purification procedure, and the environmental impact of their synthesis and use is high. Therefore, inorganic cells still cannot provide cost-efficient alternative to other ‘green’ energy sources, such as hydropower.
Recently, solar cells based on organic materials have been developed, although to date they are far less efficient than silicon based cells. Advantages of organic solar cells are that they are lightweight and more environmentally friendly. There is no need for rare metals and minerals, and no need for high temperatures and purity at the production stage. Thus, inorganic solar cells are potentially very inexpensive to make and there is virtually unlimited room for further material modification and improvement.
Operation of organic solar cells is mechanistically more complex than that of inorganic solar cells. First, a molecule of an organic compound absorbs a photon and forms an excited state (exciton). Further, the exciton diffuses to a junction border between n- and p-types of semiconductor where it dissociates to form free charge carriers. Organic p- and n-transporters are also known as donors and acceptors correspondingly. If there is no junction border nearby, the exciton may recombine (decay) via photoluminescence, or thermally, back into the ground state of the molecule. This non-productive decay pathway is the main reason that mono- and bilayered organic solar cells were poorly performing devices, until the new concept of bulk heterojunction cells was introduced.
An organic bulk heterojunction cell consists of two electrodes (anode and the cathode) which sandwich the photoactive layer of a device (see FIG. 1). A bulk heterojunction is a tight blend of a p-type conductor (donor), and n-type conductor (acceptor) in the photoactive layer of a device, where the concentration of each component often gradually increases when approaching the corresponding electrode. This affords vast expansion of p-n-junction's total surface and strongly facilitates the exciton's dissociation into positive (holes) and negative (electrons) charges. The anode and the cathode collect the electrons and the holes respectively. The anode is usually coated with an electron transport layer to facilitate the movement of electrons to this electrode. Similarly, the cathode is maybe coated with a hole transport layer which facilitates the collection of positive charges (holes). The implication of this concept in practice allows increase of power conversion efficiencies of up to 5% for all-organic solar cells. The entire device is encapsulated using a suitable material to prevent leakage of moisture and oxygen.
In spite of the impressive results achieved with the realization of the bulk-heterojunction concept, the organic cells and materials still need to be strongly improved in order to find commercial application. Current organic solar cells exhibit low power conversion efficiency, about 4%-5%, compared with silicon based solar cells, even after sophisticated device optimization. In fact, the best efficiency achieved to date has been about 8% efficiency, but these are lab-based results only and, therefore, are not generally achievable, and there is always room for further improvement.
Conjugated polymers have shown some promise in providing a photovoltaic effect. Conjugated polymers are made of alternating single and double carbon-carbon (C—C) or carbon-nitrogen (C—N) bonds. The conjugated polymers have a δ-bond backbone of interesting sp2 hybrid orbitals. The pz orbitals on the carbon atoms overlap with neighboring pz orbitals to provide π-bonds. The electrons that comprise the π-bonds are delocalized over the whole molecule. These polymers exhibit electronic properties similar to those seen in inorganic semiconductors. The semiconducting properties of the photovoltaic polymers are derived from their delocalized π-bonds.
WO03032072, as an example, describes a number of highly conjugated polymers that exhibit photovoltaic effects. Some of those compounds include thieno[3,4-b]thiophene related compounds, including octyl-6-dibromo-3-fluorothieno[3,4-b]thiophene-2-carboxylate. Thieno[3,4-b]thiophene derivatives, such as TT and FTT, are conjugated molecules that show a photoelectric effect and have the following chemical structures:

Yongye Liang, et al., Development of New Semiconducting Polymers for High Performance Solar Cells, J. AM. CHEM. SOC. 131 (2009a) pp. 56-57, for example, describes a highly processible and stable semiconducting polymer called PTB 1, based on alternating thieno-[3,4-b]thiophene and benzodithiophene monomers. Simple solar cells prepared from the blend of this polymer and fullerene derivatives exhibit a high solar energy conversion efficiency of 5.6% and high fill factor of over 65%.
Yongye Liang, et al., Highly Efficient Solar Cell Polymers Developed via Fine-Tuning of Structural and Electronic Properties, J. AM. CHEM. SOC. 131 (2009b) pp. 7792-7799 developed further variations of the above polymer, as follows:

These polymers were mixed with PC61BM or PC71BM and simple solar cells were made and evaluated. The HOMO energy levels of the polymer were lowered by substituting alkoxy side chains to the less electron-donating alkyl chains in PTB3 or introducing electron-withdrawing fluorine into the polymer backbone in PTB4, leading to significant increase in VOC (28%) for polymer solar cells. The side chains and substitute groups also affect the polymer's absorption and hole mobility, as well as the miscibility with fulleride, all influencing polymer solar cell performances. Films prepared from mixed solvent exhibit finely distributed polymer/fulleride interpenetrating network and show significantly enhanced solar cell conversion efficiency.
A power conversion efficiency of over 6% was achieved in solar cells based on fluorinated PTB4/PC61BM composite films prepared from mixed solvents. However, the synthesis of the fluorinated monomer was substantially less than satisfactory. The fluorine was introduced to the fused ring unit from 4,6-dihydrothieno[3,4-b]thiophene-2-carboxylic acid after deprotonation by using BuLi and reacting with PhSO2NF. However, this reaction produced only a mixture containing fluorinated product and unfluorinated reactant with a 4:1 ratio and a yield of only 65%, and which still needed to undergo further reaction to yield the dibrominated monomer at an overall yield of about 25%. See also US20110124822 and WO2010008672. Thus, it is expected that the 6% PCE may be improved if the synthesis could be improved and the resulting product would be more pure.
Later in WO2010008672, also by the Yongye Liang group, the benzo[1,2-b:4,5-b′]dithiophene-fluorothienothiophene (BDT-FTT aka LY-16 therein) (see below) was shown to have reached 6.1% conversion efficiency in bulk-heterojunction (BHJ) solar cells. The improved performance was achieved by using mixed solvents in preparing polymer/fullerides spin-coating solution, which resulted in a more even morphology. Thus, the BDT-FTT/PC61BM blend film exhibited improved morphology by using dichlorobenzene/1,8-diiodooctane (97/3, v/v) as solvent. There were no large features in the TEM image of a film layer using this method.

However, as already noted, conventional methods for synthesizing BDT-FTT suffer several drawbacks. First, it is difficult to isolate the fluorinated product from non-fluorinated starting materials. Further, the overall yield of the conventional synthetic method is less than 20%, meaning the manufacturing of BDT-FTT is not economically efficient. Further, the quality of the product is less than desired, negatively affecting cell efficiency.
Therefore, there is the need for a method for synthesizing BDT-FTT with high yield and wherein one can easily isolate the product from unreacted starting materials.